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From: Floyd Davidson <>
Subject: Re: A,B,C&D tones
Date: 22 Sep 2000 00:36:55 -0800

Dr. Joshua Mikkelsen <> wrote:
> Bob Bailin <72027.3605@CompuServe.COM> wrote:
>> I was flipping thru an old Practical Peripherals 14.4K modem
>> manual the other day when I came across a reference to it
>> being able to dial the A,B,C&D tones in addition to 0-9,*,#.
>> Were A thru D ever used in the "regular" (non-PBX) phone
>> system, and would they be recognized as a real "digit" if you
>> used them on a std phone line?
>The A, B, C, and D tones were designed for use with the US
>military phone network, the Autovon.

The tones were not designed for the Autovon system, but were
indeed used by it.  (Telephones that had keypads with all of the
16 codes were in fact designed specifically for Autovon use.)

> The autovon is now the Defense Switched Network(DSN), and the
>tones have been renamed to Priority, Priority Overide, Flash,
>and Flash override, if i remember correctly.

That is the use assigned to them originally on the Autovon

>These buttons, in either case, are to allow calls to be
>prioritized; that is, a more urgent user with flash override
>ability on his pair, for instance, can interupt another call
>with his.

A priority user (on a line which is optioned to allow it) could
cause the network to preempt a busy trunk if there were no idle
trunks available.  On the commercial Public Switched Telephone
Network (PSTN) callers rarely ever get an All Trunks Busy (ATB)
except during an outage when few or no trunks are available.
(The classic example though, is Mother's Day, when the traffic
is far more than the network can handle.)  The Autovon system,
however, was not provisioned to be nearly that available.  Plus
during a true emergency it can be expected that a much higher
than normal call load will be experienced and that effective use
by emergency co-ordinators will be blocked.  Hence the priority

Among those less aware of the technical aspects of the Autovon
network it was always commonly thought that priority preemption
was very common, but in fact is was exceedingly rare. Even a SAC
base might have had only one or two lines capable of using
preemption, and then only by the three or four highest ranking
operations officers (which would exclude, for example, the base
commander but might include a bomber wing commander).  The
reason for that difference between reality and perception was
because the autovohn system was notoriously under trunked and
equally notoriously poorly maintained.  Hence ATB's and being
cutoff were both a very common occurance for users, and every
Airman Second Class wanted to believe that the reason his call
went "click" and was gone was because a 4 star general had hit
the Flash Override putton and preempted him!


Floyd L. Davidson                
Ukpeagvik (Barrow, Alaska)

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